HTTP/2 Draft 16

Motivation:
HTTP/2 draft 16 has been released https://tools.ietf.org/html/draft-ietf-httpbis-http2-16.

Modifications:
The HTTP/2 codec should be updated to support draft 16.

Result:
HTTP/2 codec is draft 16 compliant.
@nmittler - Another comment that was collapsed...

From you:
>I think this comment is out-of-date ... it's not creating a stream.

From me:
>I'm not sure it is out of date and I think it will potentially create a stream if there is no stream associated with streamDependency.

>See https://github.com/netty/netty/pull/3217/files#diff-9d991ca64301bcd5b3c00d053591f485R373.